1. Field of the Invention
The present invention relates generally to repeaters for transmitting received digital audio signals to external apparatus such as DATs (Digital Audio Tape Recorders) and, more particularly, to a repeater of a digital audio interface signals employing a serial copy management system (hereinafter referred to as "SCMS").
2. Description of the Related Art
Digital signal processing has made a remarkable progress in recent years.
For example, a CD (Compact Disc), on which a digital signal capable of reproducing music or the like by optical reading is recorded in advance, has replaced a record that had played a leading role as a recording medium. It appears that this substitution results from the excellent quality of a sound reproduced by digital signal processing, the superior operability of digital apparatus and so on.
A DAT has also been developed that can record a signal onto a tape as well as reproduce a signal, from the tape by employing a digital signal similar to the CD. Use of the DAT makes it possible to make any number of copies of the CD, music tapes and so on (hereinafter abbreviated as "music software"), which are now commercially put on the market, without any degradation in quality of recording.
Because of its excellent performance, the DAT has been expected to create a new demand in the music industry. However, the DAT is hardly popular at the present time. A main reason for this lack of popularity is that strong opposition arose from the music software industry i.e., the industry which supplying CDs and music tapes in the market, or music performers.
Since the DAT employs a digital signal, it enables copying of music software without any degradation in quality of recording. Further, the DAT has enhanced recording/reproduction functions using signal interleave or the like.
As a result, if there is at least one sound source supplied through a proper route, a plurality of copies can be made serially with the sound source used as a starting point. If an original sound source is recorded by employing a digital signal, its copy is made by recording a signal with identical quality as that of a proper sound source. When the sound source has information recorded by employing an analog signal, its initial copy is slightly lower in quality of recording than the sound source, whereas the subsequent copies are made by employing a digital signal, resulting in no degradation in quality of recording.
Such copying, even if it is private, infringes a copyright of an author or the like of the original sound source (17 U.S.C., Sec. 160 (1), 501 (a)).
When the DAT becomes popular, the above-described improper copies are liable to be distributed. These improper copies have the same quality as that of the original. Consequently, the DAT is liable to promote infringement of copyrights, neighboring rights, etc. of composers, song writers, performers or record manufactures.
The foregoing is a principal opinion of people who are against the distribution of the DAT. It cannot be denied that the aforementioned anxiety has sufficient roots.
Meanwhile, the excellent sound quality and superior operability of the DAT is a great attraction to those who demand the DAT. Manufactures and suppliers of hardware such as DAT decks also expect that the DAT will create a large market. Thus, the industry for supplying hardware and the industry for supplying music software have made an agreement on a system for determining the handling of music software in the DAT in their a recent conference. This system is called a serial copy management system (SCMS).
The SCMS adopts such a method that a copy made from DAT or the like employing a digital signal from a digital sound source (hereinafter referred to as "digital copy") is limited to one generation in principle. Principal contents of this method are given below:
(1) A field for designating whether a copy is allowed or not (hereinafter referred to as copy bit) is provided on a predetermined format of a digital signal to be input to DAT (Such digital signal hereinafter referred to as "digital audio interface signal".) A DAT deck determines whether a digital copy is allowed or not, with reference to the contents of the copy bit when recording a digital signal on a tape. The DAT deck newly writes a signal indicating whether a digital copy is allowed or not into a reproduced signal, if necessary, upon signal reproduction.
(2) A digital copy from a digital source including a CD is prohibited in principle. For users who want to avoid reproduction from an original source as much as possible, a digital copy is allowed with only one generation with respect to a specific source included in a table called a "white list". This principle is adopted to all sources except for a digital source that manifestly includes a copy allowing signal.
(3) It is allowed that an analog signal obtained by reproduction of a record or a CD is digital-copied onto a tape by a DAT deck via an A/D (analog/digital) converter. In this case also, however, only one generation of copying is allowed from the copied tape.
(4) No limitation is placed on the copying by an analog signal alone.
In order to realize the foregoing contents, the SCMS method utilizes information for controlling signal processing, which is called channel status information, in the digital audio interface signal. The channel status information includes a "category code" indicating the type of a sound source information such as of a copy bit indicating whether copying is allowed or not. A determination is made as to whether digital copying of a digital audio interface signal is allowed or not in accordance with both information of the category code and copy bit.
When a copy from a sound source is made, the DAT deck takes out the aforementioned channel status information in an input signal and then determines whether or not copying is allowed based on the category code and copy bit. In reproduction from a tape, the DAT deck, if necessary, changes, e.g., the category code in a reproduced signal to one that is not shown on the white list, thereby to prevent a subsequent copying.
With reference to FIG. 1, a digital audio interface signal 52 output from a digital signal source 12 is conventionally input via a digital signal repeater 34 or directly to a DAT deck 14. DAT deck 14 detects per se whether or not copying of the digital signal to be input is allowed based on the channel status information.
DAT deck 14 is programmed to perform an appropriate operation in accordance with the channel status information when the copying by the digital signal is instructed by an operator. More specifically, DAT deck 14 puts itself into a recording state when the copying of the digital signal to be input is allowed, whereas it does not when the copying of the input signal is prohibited.
The digital copying employing by the SCMS method has a plurality of allowing patterns, several examples of which are shown in FIG. 2. These patterns are grasped by the concept of "generation management".
With reference to FIG. 2 (a), a DAT deck 14a can copy onto a tape an analog signal reproduced from an LP record or an analog signal obtained by reproduction of a digital sound source such as a CD. This process corresponds to the above-described case (3). Thus, one more digital copying can be made from the copied tape.
In further detail, DAT deck 14a writes a control signal into a record signal with reference to channel status information upon digitizing an analog signal and then copying the digitized signal. A category code of the control signal in this case indicates "the first digital copying from an analog sound source".
Such a case will now be considered that this tape is further digital-copied by a DAT deck 14b. This digital copying is allowed since it corresponds to the above case (3). According to the foregoing determination, information indicating "a digital copying of the second generation from the analog sound source" is recorded on a control signal in a newly copied tape.
Another case is now considered that this newly copied tape is to be further digital-copied by a DAT deck 14c. It is necessary to reproduce the tape once more in order to make a copy. In this reproduction, a control signal is also reproduced. Since this tape is the second-generation copy as described above, a deck for reproducing this control signal rewrites the contents of channel status information of an output signal, so as not to allow further copies to be made. Thus, even if DAT deck 14c is instructed to make a digital copy of a reproduced signal, the deck is not allowed to make the digital copy.
As shown in FIG. 2 (b), a digital signal to be output from a CD player or the like can be digital-copied only once by DAT deck 14a according to the principle (1). Upon this copying, information indicating "the first digital copying from a digital sound source" is recorded in a control signal according again to the principle (1). It is impossible to make another copy tape from this copied tape by DAT 14b according to the principle (1).
A case when no digital copying can be performed may occur with reference to FIG. 2 (c). One example of this case is that one borrows a tape which is digital-copied from a digital signal source by another person and attempts to make a further digital copy of this copied tape. Alternatively, such a case is also considered that one makes a digital copy of a digital sound source which is unlisted on the white list.
With reference to FIG. 2 (d), a digital audio interface signal, in which a copy bit thereof indicates "copying is allowed", can undergo any subsequent generations of digital copying.
The digital audio interface signal employed in the SCMS method has the following format. Referring to FIG. 3, the digital audio interface signal is divided into a plurality of blocks. Each block includes 192 frames.
Each frame includes two subframes each including 32-bit data. With reference to FIG. 4, an indicator called a preamble for indicating what type of information is recorded on a subframe is located at the head of each subframe.
In a 2-channel audio signal, for example, one of the subframes belongs to a channel one, while the other subframe belongs to a channel two. The respective subframes of channels one and two are distinguished from each other by their respective preambles. That is, the preamble of the subframe of channel one is a signal denoted with the symbol "M", and that of channel two is a signal denoted with "W".
The preamble of the first subframe of the leading frame in one block is a specific preamble (denoted with "B"). Synchronization of the blocks is carried out by detection of the specific preambles "B".
With reference to FIG. 4, the 31st bit (bit 30) of each subframe of channel one is used for recording of the above-described channel status information. One block includes 192 subframes of channel one. Use of bit 30 of each subframe one enables recording of 192-bit channel status information per block. The channel status information is recorded in order from the head of the block.
In FIG. 4, only the preamble and the bit for channel status information are shown in the format of the subframe. A description as to other data is omitted so as to further facilitate the description.
FIG. 5 shows a format of channel status information. The DAT deck detects channel status information from a digital audio interface signal to be input and carries out various processing according to the channel status information.
Referring to FIG. 5, bits 1-5 of the channel status information are called "control bits". The control bits represent an attribute of the digital audio interface signal.
Referring to FIG. 6, combinations of each bit of the control bits indicate various meanings. Bit 2 of the control bits is a copy bit representing information that copying is allowed.
When the copy bit (bit 2 in FIG. 6) is "0", digital copying is inhibited. When the copy bit is "1", the digital audio interface signal can be digital-copied any number of times.
When the copy bit is "0" as mentioned above, digital copying is inhibited in principle. However, there is a case where digital copying is allowed only once in accordance with a category code represented by bits 8-15 of FIG. 5. The detail of the category code is shown in FIG. 6A.
The category codes, which allow only a single digital copying, are listed on the white list as named before. With reference to FIG. 6A, for example, when the category code is "10000000" (CD), "11000001" (DAT-P) and so on, digital copying is allowed only once.
When making a digital copying from a sound source of the category code included in the white list, the DAT deck alters the category code of a signal to be copied into "11000000" (DAT). Since the DAT is unlisted on the white list, subsequent copying is inhibited.
As aforementioned, checking the copy bit and the category code in the channel status information makes it possible to know whether or not copying of the digital audio interface signal being under processing is allowed.
The SCMS method aims to place a limitation on a digital copy from a digital source such as music software, as described above, thereby to prevent serial, unlimited, unauthorized copying of music software. It is expected that adoption of the SCMS method to the DAT deck advances the distribution of the DAT.
However, the DAT deck in accordance with the SCMS method has the following disadvantages.
With reference to FIG. 2, the digital copying is sometimes allowed but sometimes not, as has already been described above. Even when the copying is allowed, various cases may occur with respect to how many generations the subsequent copying is allowed.
In a conventional digital signal repeated or a conventional DAT deck, however, a determination cannot be made easily as to whether a digital audio interface signal being currently processed can be copied or not. For example, no determination can be made as to whether or not the copying of a digital signal under processing is allowed, unless such as attempt is made to put the DAT deck into the recording state. Thus, the conventional digital signal repeated or DAT deck is extremely inconvenient in digital copying.
Moreover, it is not easy to know information as to how many copies can further can be made from a copied tape. This results in poor operability in digital recording.